The present invention relates to spheroidal graphite cast iron having excellent mechanical strength and a method of producing such spheroidal graphite cast iron.
Since spheroidal graphite cast iron has excellent mechanical strength, it is widely used in various applications including automobile parts, machine parts, etc. Recently, spheroidal graphite cast iron containing a large amount of sulfur was proposed ("Addition of Rare Earth Elements in the Production of Spheroidal Graphite Cast Iron and Its Practice," JACTNEWS No. 341, pp. 22-28, published by the Chuzo Gijutsu Fukyu Kyokai (the Casting Technology Association), and Japanese Patent Laid-Open No. 61-15910). By this technology, it was made possible to produce spheroidal graphite cast iron having excellent graphitization ratio even by cupola melting.
In general, in the cupola melting method, spheroidal graphite cast iron containing a large amount of sulfur is used, making it necessary to add a large amount of a spheroidizing agent. Thus, the production costs of spheroidal graphite cast iron increase. Further, the spheroidal graphite cast iron produced by cupola melting has a relatively small number of spheroidal graphite particles per unit area, resulting in poor mechanical strength.
On the other hand, in the production of spheroidal graphite cast iron by means of an electric furnace, a melt is desulfurized such that sulfur content is 0.015% or less, or a starting Fe alloy material containing such a small amount of sulfur is used. The spheroidal graphite cast iron can be produced by using a smaller amount of a spheroidizing agent in the case of a low sulfur content melt than in the case of a high sulfur content melt.
To conduct the desulfurization of a melt, a desulfurizing agent such as carbide should be added, and since the melt is subjected to gas blowing, stirring, etc., there should be facilities for preventing the decrease in melt temperature and for preventing melt scattering. These problems cause the deterioration of environment and increase the production costs.